The passwords sheet of the keyboard accessory currently only allows filling passwords and is only shown for password fields. We should consider to offer filling the username as well.

Challenges:
1. Figure out when to show the keyboard accessory.
On search fields and in social media text fields, the accessory might be confusing. Launching a "reliable fallback method" on some fields only might not be better though.

2. It might uncover some currently hidden problems like issue 888933.